1. Field of the Invention
The present invention relates to an adjustable harmonic distortion detector and method for using the same, particularly for linear (AB class) and switching (D class) amplifiers.
2. Description of Related Art
The term distortion designates any undesired deformation in the time trend of a signal and we distinguish two types of distortion: a) linear; and b) non linear.
The linear distortion happens when an input sinusoidal signal having a given amplitude and a given phase causes a still sinusoidal output signal having a different amplitude and/or phase.
The non linear distortion happens when an input sinusoidal signal causes an output signal which is no longer as such.
Particularly, in this last case, the distortion is measured in function of the deformation that the sinusoidal signal suffers, being founded on the possibility of dividing the deformed signal, into a sinusoidal oscillation having the same frequency (called fundamental oscillation) and into a number “n”, with “n” rising to infinity, of oscillations having multiple frequencies of the fundamental (called harmonic oscillations).
We assume as convention, as a measure of the total harmonic distortion T.H.D., the ratio, usually expressed percent, between the total efficacious value of the harmonics and the efficacious value of the fundamental.
The distortion entity has a particular importance in the acoustic frequency amplifiers, that is for audio amplifiers, because it can modify the signal intelligibility.
An usual function in the audio amplifiers is the so called distortion detector function, called “clipping function”, that has the role to detect when an amplifier has reached the peak power.
Using this information we can limit the distortions that a signal outgoing from the amplifier suffers, distortions due to the reached threshold of saturation of the same amplifier.
In this way by means of opportune circuits we can define “a priori” a fixed level of acceptable distortion, that is we can define at which distortion level the clipping function can interfere so as to reduce the amplifier gain, limiting, therefore, the distortion inside desiderated ranges.
The circuits now used are essentially of three categories: a) by a sensor of the saturation of the output power transistor of the audio amplifier, able to detect a distortion at a prefixed threshold value, as shown in FIG. 1 wherein we note an ideal output signal 1, a distorted signal 2 and a prefixed intervention threshold 3; b) by a comparison between the peak voltage of the input signal and the peak voltage of the same signal at the output of the audio amplifier when this last output signal is distorted, as shown in FIG. 2; c) by a measurement of the unbalance of the input pins of the audio amplifier (in fact during the normal working the input pins are at the same voltage, while when the amplifier saturates the pins assume a voltage difference that depends on the input signal amplitude).
The circuits heretofore described work correctly only in the case of AB class amplifiers, give troubles if the amplifier is D class and mostly if the amplifier is implemented in an integrated circuit wherein the output signal is not available inside the same integrated circuit.